Table header/footer views

@segiddins @andrewsardone -- `UITableHeaderFooterView` in `SSSectionedDataSource`!

* Moved `SSSection` to its own header/impl. files
* New `SSBaseHeaderFooterView` that handles reuse identifier for you
* See the example sectioned table for sample usage

Ultimately there's not a lot going on here. No block-based creators, no multiple header/footer view classes, and I don't even get into registering header/footer classes with the table.

My thinking is that almost all of this falls under `UITableViewDelegate` and is therefore somewhat outside the scope of what I meant for `SSDataSources`. Is there enough here to be useful?